1. Field of the Invention
This invention relates to a process for producing a 5-alkylidenenorbornene by the isomerization of a 5-alkenylnorbornene.
2. Description of the Prior Art
5-Alkylidenenorbornenes are industrially important diene monomers as the third components of ethylene-propylene-diene rubbers (hereinafter referred to as EPDM). Above all, 5-ethylidenenorbornene (hereinafter referred to as ENB) is most widely used.
The production of ENB by the isomerization of 5-vinylnorbornene (hereinafter referred to as VNB), which is obtained by the Diels-Alder reaction from butadiene-1,3 and cyclopentadiene, is commerically advantageous because of easy availability and inexpensiveness of the starting materials. The isomerization of VNB can be effected by contacting with either acidic or basic reagent. However, since the acidic reagent does not give a good yield of ENB owing to partial polymerization and the like, a basic reagent is used in many cases. For instance, there is known a process in which VNB is contacted with (a) an alkali metal supported on a carrier, (b) a mixture of a strong base of an alkali metal and a polar organic solvent, or (c) a mixture of an alkali metal amide and a nitrogen-containing base (British Pat. No. 1,111,924). However, in view of disaster prevention, direct use of an alkali metal such as sodium, potassium or lithium in this method has a disadvantage of dangerous handlings.
In another known method, the isomerization of VNB into ENB is effected in the presence of a ternary system catalyst comprising an alkali metal or a complex thereof with ammonia, a carrier having a large surface area, and a compound of an alkali metal or alkaline earth metal (British Pat. No. 1,335,138). However, in this method too, there is a danger associated with the use of an alkali method as in the above method. Further, the procedure of supporting an alkali metal is troublesome and dangerous and a complex of an alkali metal with ammonia such as sodium amide is expensive.
In order to eliminate the above disadvantages, methods have been proposed for isomerizing VNB into ENB by contacting VNB with a mixture of an alkali metal hydride and dimethyl sulfoxide (Japanese Patent Publication No. 24,388/73) or a mixture of an alkali metal hydride and an aliphatic amine (Japanese Patent Publication No, 35,072/75). These methods provide an improvement in the conventional methods by utilizing an alkali metal hydride which is available at low cost and can be handled with relative easiness. These methods, however, have drawbacks in that they require a large quantity of comparatively expensive dimethyl sulfoxide or aliphatic amines and the repeated use of the catalyst is difficult because of the solubility of the catalyst in these reagents. A further disadvantage is a considerably high cost of recovering the reagent if an aliphatic amine having a boiling point relatively close to that of VNB or ENB is used.
In other methods than the above-mentioned methods of isomerization with alkali metals, there is used as catalyst a metal pentacarbonyl (Japanese Patent Publications Nos. 23,337/70 and 24,478/74) or a combination of an organic alkali metal compound and a polyamine (Japanese Patent Publication No. 40,469/74). Such a method has a disadvantage of high cost of the catalyst and, hence, of the isomerization.